Heart disease is the number one cause of death and disability in women in the United States. Research studies suggest that estrogen replacement therapy after menopause reduces the risk of heart attacks in women. However, it is not clear how estrogen works to reduce heart attack risks. There is reason to believe estrogen may prevent coronary artery blockages (hardening of the arteries around the heart) from getting worse or even cause them to get better. Recent research from our institution demonstrates that estrogen may also help protect the inner lining of blood vessels, called the endothelium, from the effects of coronary artery disease (atherosclerosis or hardening of the arteries). If these effects can be proven in women, it will significantly advance our understanding of how to prevent heart attacks. Given the magnitude of the problem of heart disease in postmenopausal women, and the paucity of primary and secondary prevention trials of heart disease in women, verifying the presumed benefit of estrogen replacement therapy is one of the most pressing national health policy issues of this decade. Furthermore, the strength of the apparent cardioprotective effect of estrogen in observational studies, if real, implies that estrogen plays a fundamentally important role in the maintenance of vascular health -- one that if better understood could significantly advance our understanding of atherosclerosis and how to reduce or prevent it. The ERA trial will use quantitative coronary angiography and measures of endothelial function to determine the effects of estrogen replacement therapy on the coronary anatomy and progression of atherosclerosis in arteries of postmenopausal women.